1. Field of the Invention
The present invention relates to an EL (Electro Luminescence) display panel, an electronic apparatus, and a method of driving the EL display panel, and more particularly to an EL display panel which is driven and controlled by using an active matrix drive system, an electronic apparatus, and a method of driving the EL display panel.
2. Description of the Related Art
FIG. 1 shows a general circuit block of a configuration of an active matrix drive type organic EL display panel. As shown in FIG. 1, an organic EL display panel 1 is composed of a pixel array portion 3, a signal write control line driving portion 5, and a horizontal selector 7 which operate as drive circuits for driving the pixel array portion 3. It is noted that in the pixel array portion 3, a pixel circuit 9 is disposed in each of intersections between signal lines DTLs and write control lines WSLs.
Now, an organic EL element is a current light emitting element. For this reason, a drive system for controlling a gradation by controlling amounts of currents caused to flow through the organic EL elements corresponding to pixels, respectively, is adopted for the organic EL display panel.
FIG. 2 shows one of the simplest circuit configurations of this sort of pixel circuit 9. This pixel circuit 9 is composed of thin film transistors T1 and T2, and a hold capacitor Cs. Hereinafter, the thin film transistor T1 is referred to as “the sampling transistor T1,” and the thin film transistor T2 is referred to as “the drive transistor T2.”
The sampling transistor T1 is an N-channel thin film transistor for controlling an operation for writing a signal potential Vsig corresponding to a gradation of the corresponding one of the pixels to the hold capacitor Cs. In addition, the drive transistor T2 is a P-channel thin film transistor for supplying a drive current Ids to an organic EL element OLED based on a gate-to-source voltage Vgs determined depending on the signal potential Vsig held in the hold capacitor Cs.
In the case of the circuit configuration shown in FIG. 2, a source electrode of the drive transistor T2 is connected to a power source line to which a power source potential Vcc is fixedly applied, and thus the drive transistor T2 usually operates in a saturated region. That is to say, the drive transistor T2 operates as a constant current source for supplying a drive current Ids having a magnitude corresponding to the signal potential Vsig to the organic EL element OLED. In this case, the drive current Ids is expressed by Expression (1):Ids=k·μ·(Vgs−Vth)2/2  (1)where μ is a mobility of a majority carrier of the drive transistor T2, Vth is a threshold voltage of the drive transistor T2, and k is a coefficient given by (W/L)·Cox where W is a channel width, L is a channel length, and Cox is a gate capacitance per unit area.
It is noted that in the case of the pixel circuit having this configuration, it is known that there are the characteristics in which a drain voltage of the drive transistor T2 changes with a temporal change of I-V characteristics of the organic EL element shown in FIG. 3. However, since the gate-to-source voltage Vgs is held constant, there is no change in amount of current supplied to the organic EL element. As a result, an emission luminance can be held constant.
The organic EL display panel device adopting the active matrix drive system, for example, is described in Japanese Patent Laid-Open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.